TITLE:   GCN CIRCULAR
NUMBER:  29595
SUBJECT: GRB 210305A: Swift detection of a burst
DATE:    21/03/05 19:21:53 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

A. D'Ai (INAF-IASFPA), E. Ambrosi (INAF-IASFPA), V. D'Elia (SSDC),
N. J. Klingler (PSU), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester) and
D. M. Palmer (LANL) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 19:03:04 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 210305A (trigger=1035922).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 319.814, +34.557 which is 
   RA(J2000) = 21h 19m 15s
   Dec(J2000) = +34d 33' 24"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
structure with a duration of about 100 sec.  The peak count rate
was ~3200 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 19:04:21.28 UT, 76.8 seconds after
the BAT trigger. Using promptly downlinked data we find an
uncatalogued X-ray source with position: RA, Dec 319.80764, 
34.554389 which is equivalent to:
   RA(J2000)  = 21h 19m 13.83s
   Dec(J2000) = +34d 33' 15.8"
with an uncertainty of 4.7 arcseconds (radius, 90% containment). This
location is 21 arcseconds from the BAT onboard position, within the BAT
error circle. This position may be improved as more data are received. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 85 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of
the XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag. 
The 8'x8' region for the list of sources generated on-board covers 100% of the
XRT error circle. The list of sources is typically complete to about 18 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.155. 

Burst Advocate for this burst is A. D'Ai (antonino.dai AT inaf.it). 
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/)